On the evening of August 21, 1986, a catastrophic event unfolded in the remote northwestern region of Cameroon. A silent, invisible force moved down from the waters of Lake Nyos, a crater lake nestled in a volcanic plain. This phenomenon swept through the surrounding valleys and by the next morning, left a trail of devastation. Over 1,700 people and more than 3,500 livestock perished, not from a violent explosion or earthquake, but from a silent release of gas that left buildings and vegetation untouched.
The Eruption of August 1986
The disaster began around 9:30 p.m. with a rumbling sound that some survivors reported hearing from the direction of the lake. This was followed by the emergence of a large, white cloud that rose hundreds of feet into the air before sinking back down. Being heavier than air, this dense cloud hugged the ground and flowed swiftly down the valleys, channeled by the terrain into the nearby villages of Nyos, Kam, Cha, and Subum. The gas cloud traveled at speeds estimated between 12 and 31 miles per hour.
Those who heard the sounds and went to investigate lost consciousness almost immediately. Many others were overcome in their sleep, never waking up. The cloud displaced the breathable air in its path, which extended up to 15.5 miles from the lake. In the village of Nyos, closest to the source, only four residents survived, all of whom were on higher ground. For up to 36 hours, many survivors remained unconscious before waking to a silent landscape littered with the bodies of their families, neighbors, and animals.
The Science of a Limnic Eruption
The event at Lake Nyos was a rare natural disaster known as a limnic eruption, or lake overturn. Lake Nyos sits in a volcanic crater, and beneath it lies a pocket of magma that slowly leaks carbon dioxide (CO2). This gas percolates up through the Earth’s crust and dissolves into the cold, deep bottom waters of the lake. Over many years, this process allowed an immense amount of CO2 to accumulate in the lake’s lower layer, called the hypolimnion.
Normally, lakes undergo seasonal turnover, where surface and bottom waters mix, preventing such a buildup. However, Lake Nyos is a meromictic lake, meaning its layers are sharply stratified and do not mix. The cool, dense, gas-rich water at the bottom was held in place by the warmer, lighter water at the surface, creating a high-pressure container for the dissolved CO2.
On that August night, this stable stratification was disrupted. The exact trigger is unknown, but scientists suspect a landslide, a small volcanic event on the lakebed, or cool rainwater disturbing the surface layer was the cause. This disturbance acted like opening a shaken bottle of soda; the deep, supersaturated water rose, and the reduction in pressure allowed the dissolved CO2 to erupt out of the solution. An estimated 1.2 cubic kilometers of CO2 gas was released. The normally blue lake water turned a deep red as iron-rich bottom water was brought to the surface and oxidized.
The Devastating Consequences
When the outside world learned of the disaster, rescue teams and scientists arrived to a baffling scene. They found the bodies of 1,746 people and roughly 3,500 cattle and other animals strewn across the landscape. The sheer scale of the silent death toll initially confused first responders.
The few thousand survivors who were evacuated from the area reported symptoms consistent with CO2 poisoning and oxygen deprivation, known as asphyxia. Many experienced respiratory problems, confusion, and temporary paralysis. Some survivors had skin lesions, later determined to be pressure sores caused by lying unconscious for extended periods due to a lack of oxygen in their blood.
Degassing and Modern Monitoring
After the disaster, an international effort was launched to prevent a recurrence. The solution was a system to safely remove the accumulating CO2 from the lake’s depths. In 2001, the first degassing pipe was installed, running from the gas-saturated bottom layer to the surface. A pump was initially used to draw the deep water up the pipe. As the water rises and the pressure decreases, the dissolved CO2 comes out of solution, creating a fountain effect that makes the system self-sustaining.
This process allows the trapped gas to be released in small, controlled amounts that dissipate harmlessly into the atmosphere. Two additional pipes were installed in 2011 to increase the degassing rate, further reducing the CO2 concentration in the lake. The lake is now much safer but requires continuous monitoring. This technology has also been applied to Lake Monoun, another Cameroonian lake that experienced a similar event in 1984. Scientists also monitor Lake Kivu in the Democratic Republic of the Congo, which holds an even larger reservoir of dissolved gases.